The present invention relates to a method for determining the quality of haemoglobin (Hb) during the storage period of red blood cell (RBC) units comprising a step of detecting soluble alpha-haemoglobin (?-Hb) pool in RBC lysates and concluding that the presence of ?-Hb pool indicates a conservation of quality of Hb during the storage RBCs. rnInventors have determined the impact of RBC units aging on the quality of Hb and on the soluble ?-Hb pool level in RBCs. For this purpose, 21 RBC units were collected, stored at +4 to 6°C and samples were taken at two different storage times (D3-D8 and D38-D42) to evaluate spectral characteristics of Hb and soluble ?-Hb pool in RBCs. Two additional samples were collected from 16 RBC units, at intermediate time points during storage (D13-D17 and D24-D29; n = 16). The ?-Hb dosing assay uses the specific character of the interaction between the ?-Hb and the AHSP, the ? chaperone, to trap the ?-Hb present in the RBC lysates of patients. They also investigated the effect of a short cryopreservation period at -80°C for 15 days on the ?-Hb pool for 4 different RBC units.rn